This post is from my series on building a backyard foundry.
My third mistake was building the furnace before I had obtained the crucible. Post construction I went to a number of thrift stores looking for cast iron pots, tall stainless steel tubs, and so on, to try to find something that would fit the 6 3/4 inch bore of the furnace. It would have been better to obtain a good crucible first, and then ensure that the furnace fits it. Remember, for a charcoal furnace you have to be able to pack charcoal around every side of the crucible; the maximum outer diameter of the crucible should be about 2 inches less than the bore of the furnace.
Thus far I’ve been using a cheap 4 inch diameter stainless steel tub made of pretty flimsy steel. Though it has almost no thermal mass and therefore heats up red hot very quickly, the thin steel will (1) be dissolved by the molten aluminum on the inside, and (2) will oxidize on the outside, and will eventually fail. I’ve therefore obtained a 3-inch inner diameter black steel pipe nipple and pipe cap from Ballard Hardware. It needs some modifications before it will be a useful crucible; more on that later. I wish I had obtained the crucible first, because then I would have chosen the larger 4-inch inner diameter nipple, which does not fit well in the bore.
My fourth mistake was one of operation, and was very simple to fix: I did not use nearly enough charcoal the first couple of times I tried to melt aluminum. Once I started putting in a good four inches or so in the bottom, and more on the sides, it melted nicely.
My fifth mistake was also one of operation: when I went to go for my first pour, when the metal actually melted, I got too excited and poured too soon. I was not molding anything other than ingots in a muffin tin; the metal froze in the crucible when halfway poured into the muffin tin. The combination of pouring too soon and having not enough fuel was not good.
It is somewhat dangerous to have a mass of solid metal in the crucible, because when it heats up again, the metal will expand and possibly break the crucible. Fortunately, after I allowed it a day to cool down, the metal came out of the bottom pretty easily in a big lump. Of course, I will melt it again. One of the truly nice things about metal casting work, as opposed to, say, fine woodworking, is that you take only a small loss of materials for mistakes.
The problem of getting the metal up to pouring temperature is, once there is enough actual charcoal in there, essentially becomes an oxygen supply problem. More on that next time.